The invention relates to a method for controlling a pipe bending machine and, in particular, to a pipe bending machine for the pressure bending of pipes.
When bending pipes, a clamping jaw presses a pipe laterally against a bending template which is then turned, the clamping jaw performing a pivotal movement. When the bending template is turned, the pipe is bent around the bending template. With thin pipe walls, small bending radii, large pipe diameters and sensitive pipe materials, pressure bending is used in which a pushing device urges the unbent pipe section towards the bending template during the bending operation. Here, the feed of the pushing device is effected at a speed that is slightly higher than would correspond to the turning speed of the bending template so that, during the bending operation, the pipe is subjected to a slight upsetting in the longitudinal direction. Here, the mutual tuning between the turning movement of the bending template and the feed movement of the pushing device is of particular importance. Should the pushing device be advanced too fast or too slowly, cracks, corrugations or areas of different wall thicknesses may occur.
From German Patent 23 04 838 C2, a pipe bending device is known wherein the feed movement of the pushing device is tuned to the turning movement of the bending template. For this purpose, sensors are provided that determine the circumferential velocity and the up-setting speed from the bending angle of the bending template and the upsetting path of the pushing device. By a comparison, the difference between both velocities is formed and a servo valve is controlled in dependence on this difference, the servo valve being designed as a volume controlling valve and changing the backflow volume of the hydraulic drive of the pushing device. Thus, the measured values evaluated are velocities and the actuation signal causes a change in the rate of flow, i.e., the backflow volume of the hydraulic oil from the drive of the pushing device. It is a drawback of such a velocity control that an erroneous upsetting force once established is maintained throughout the entire pipe bending process even if the two velocities are subsequently maintained in the correct relation to each other. This means that instantaneously occurring errors are not corrected by the control system. The feed velocity of the pushing device is changed by the volume control means. However, such a flow rate control has the drawback of being comparatively inert (slow) and inaccurate and that it may occur that the flow rate predetermined by the control means is temporarily not attained because the resistance of the pushing device and the pipe is too strong. In this case, no posterior correction and no "catching up" is performed.